Process for continuous determination of paper strength

ABSTRACT

A system and process for continuously determining the strength of paper sheet material during manufacture includes a plurality of sensors for detecting proxies related to properties such as the strength of individual fibers, length distribution of fibers, quantity of fibers, distribution of fibers, orientation of fibers, number of bonds between fibers, and bond strength of fibers. For a given papermaking machine and paper type, multiple regression analysis is used to determine correlations between the measured proxies and laboratory tests of paper strength. Then, during operation of a papermaking machine, changes in the proxy measures are used to indicate paper strength and to adjust operation of the papermaking machine based upon changes in the strength of sheet material being produced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the art of papermaking and, moreparticularly, to the continuous determination of paper strength duringmanufacture of paper sheet materials.

2. State of the Art

In the papermaking industry, strength specifications are commerciallyimportant for numerous paper products including bag paper, liner board,corrugating medium, newsprint, and tissue paper. As a result of customand developments over the years, strength specifications are usuallybased upon standardized laboratory procedures for determining propertiessuch as burst strength, tensile strength, elongation, internal tearingresistance, edge tearing resistance, crush strength and so forth. Aspecific example of a widely accepted laboratory test is the Mullenburst test. A Mullen test is usually conducted by clamping a sample ofpaper across a ring and then providing a diaphragm to increase pressureagainst one side of the clamped paper until it bursts. The pressure atwhich the sample bursts is called the Mullen burst strength test.(Standard specifications for this test include TAPPI 403os-76 and ASTMD774.) Another example of a customary laboratory test procedure is the"STFI" compression test for heavy papers established by the SwedishTechnical Forest Institute. In the STFI test, a sample strip is heldbetween a pair of clamps that are moved towards each other whilecompressive force is monitored; the maximum compressive force is calledthe STFI compressive strength of the paper. (Standard specifications forthis test include TAPPI 7818os-76 and ASTM D1164.)

Still another example of a widely accepted laboratory test is thestandardized tensile strength test wherein a sample strip of paper ispulled in opposite directions with progressively increasing force untilthe sample fails; the tension at the failure point is called the tensilestrength of the paper. (Standard specifications for this test includeTAPPI Standard T404os-76 and ASTM Standard-D828.)

Laboratory test procedures in the papermaking art, however, have certaininherent limitations. One critical limitation is that substantialperiods of time are required for sample acquisition and analysis. Duringthese periods, production conditions may change sufficiently that thelaboratory tests results, when available, are no longer representativeof current manufacturing or product conditions. Another limitation isthat almost all laboratory tests detect physical failure of papermaterials and, thus, are necessarily destructive tests. Yet anotherlimitation is that laboratory tests inherently involve sampling, and therelatively small samples obtained for testing may not completely oraccurately represent sheet material that has been produced. Because ofthe above-mentioned limitations and the fact that paper qualitylaboratories can test only a small fraction of the paper produced bypapermaking machines, it often happens that enormous quantities ofsubstandard paper are produced before a quality laboratory discoversproduction problems.

In an apparent effort to automate laboratory test procedures, U.S. Pat.No. 4,550,613 suggests an apparatus for automatic determination of thetensile strength properties of a sheet of paper. The apparatus includesa cutter to cut a sample of paper of standard width and a device formeasuring the tensile strength properties of the sample.

In light of the limitations of standardized laboratory procedures,whether automated or not, workers in the papermaking art have sought tomake continuous measurements of paper strength on-line, i.e., while asheet-making machine is operating. On-line measurements, if made rapidlyand accurately, have the potential to enable nearly immediate control ofpapermaking processes and, thus, to substantially reduce the quantity ofsubstandard paper that is produced before process conditions arecorrected. In other words, on-line measurements have the potential tosubstantially reduce time delays between the occurrence and correctionof "upset" conditions in papermaking processes. In practice, however,on-line measurements of papermaking processes are difficult to makeaccurately and often cannot be well correlated with standardizedlaboratory tests.

One of the difficulties in making accurate measurements of sheetmaterial on papermaking machines arises from the fact that modernpapermaking machines are large and operate at high speeds; for example,many papermaking machines can produce sheets up to four hundred incheswide at rates, called "wire speed," of about 20 to 100 feet per second.Another complication affecting on-line measurements is that physicalproperties of paper sheet material can vary across the width of a sheetand may be different in the machine direction than in the cross sheetdirection. (Thus, in laboratory tests, paper strength typically hasdifferent values depending on whether test strips are cut in the machinedirection or the cross direction.)

Because laboratory tests of paper sheet characteristics are normallydestructive in nature, such test procedures cannot be readily adaptedfor obtaining on-line measurements. On the other hand, becausecommercial custom is such that laboratory tests of sheet properties arethe yardstick for acceptability of on-line measurements, only on-linesensors whose outputs correlate well with laboratory tests of sheetproperties are likely to have maximum acceptance in the papermakingindustry.

One specific example of a suggestion to provide on-line measurement ofmechanical properties of paper sheet materials appears in U.S. Pat. No.4,291,577, assigned to the Institute of Paper Chemistry and entitled "OnLine Ultrasonic Velocity Gauge." This patent describes a system formeasuring velocities of ultrasound waves through traveling paper websusing a device having spaced-apart wheels that roll along a travelingpaper web; the wheels have transducers on their peripheries to impartultrasound signals to the web. According to the patent, output signalsfrom the transducers can be utilized to measure the velocity of sonicwaves through the web. Also the patentee suggests that the sonicvelocity measurements can be correlated with Young's elastic moduluswhich, in turn, can be used to estimate paper strength. (See also Baum,G. A., "Paper Testing and End-Use Performance" printed in "CompressiveStrength Development on the Paper Machine", Institute of PaperChemistry, 5-8, 1984.)

Other workers in the art have also suggested that correlations existbetween tensile strength, burst strength and sonic velocity through apaper web. See, "On-line Measurement of Strength Characteristics of aMoving Sheet: Ming T. Lu, TAPPI, 58(6):80 (June 1975). Also see Seth, R.S., and Page, D. H., "The Stress Strain Curve of Paper" in "The Role ofFundamental Research in Paper Making", PIRA Symposium Proceeding,Cambridge, 1981, wherein it is reported that the elastic modulus of asheet relates to the elastic modulus of the fibers, the mean length andwidth of the fibers and the relative bonding area. Also see U.S. Pat.No. 4,574,634 that disclosed a device employing sonic transducers todetect the machine direction and cross-direction Young's moduli forpaper samples. Further, in U.S. Pat. No. 4,335,603, assigned to BeloitCorporation, it has been suggested that tension in a moving paper webcan be detected by measuring the time of travel of a sonic wave throughthe web.

By definition, Young's modulus indicates the rate of change of astress-strain relationship. In the relationship as applied to papermaterials, stress refers to loading force applied to a paper specimenand strain refers to elongation of the specimen in response to theapplied force. It has been observed that, when Young's modulus isdetermined for a given specimen of paper, the failure point of otherpaper of the same kind can sometimes be predicted. In practice, however,Young's modulus has not been rigorously related to papermaking processconditions that affect paper strength and it is known that someprocessing steps may increase the strength of paper of a certain kindwith little substantial change in Young's modulus and that otherprocessing steps, such as wet straining, may substantially affectYoung's modulus for certain kinds of paper with substantially lesseffect upon paper strength measures. See, for example, the article bySeth and Page, supra.

As further background to the present invention, it is useful togenerally describe a typical papermaking process. Broadly speaking, apapermaking process begins when a slurry of fibers and water, called rawstock, is spread from a reservoir called a "head box" onto a wire meshthat supports the web while allowing substantial drainage. After the wetweb of fibers is formed, the web is passed through a press section wherewater is squeezed from the web and then through a dryer section wherewater is evaporated from the web. After the dryer section, the webpasses through calendar rollers to provide surface finish and then,usually, through a scanner and onto a reel. The portion of a papermakingprocess prior to a dryer is often referred to as the "wet end" of theprocess. It can be appreciated that on-line measurements at the wet endare desirable because such measurements, if acted upon promptly, canprovide control early enough during paper production to allow processchanges before substantial quantities of substandard paper are produced.On the other hand, wet end measurements are difficult to make because ofthe high water content of paper webs at this stage and because offrequently severe environmental conditions.

Still further as background to the present invention, it should beunderstood that papermaking machines have been instrumented to includesensors to detect parameters such as wire speed, basis weight, moisturecontent, and caliper of the paper during production. Many of the on-linesensors are designed to periodically traverse or "scan" traveling websof sheet material to provide successive measurements across the webs.(In the sheet-making art, a succession of measurements at adjacentlocations that, in total, spans a traveling web in the cross directionis called a "profile.") Scanning systems are advantageous because, asmentioned previously, various properties of paper may vary across asheet as well as along the sheet; particularly, cross-direction strengthproperties may be different than machine direction strength properties.

Examples of scanning systems are provided in U.S. Pat. Nos. 3,641,349;3,681,595; 3,757,122; and 3,886,036 assigned to Measurex Corporation.Other specific examples of scanning gauges proposed by workers in theart include ones that detect the composition of sheet material bymeasuring the radiation absorbed from beams of infrared light or otherradiation of known wavelength directed against a given area of the sheetmaterial. Devices of the latter type operate in accordance with thegeneral principal that the amount of radiation absorbed by sheetmaterial at a particular wavelength is a function of the composition ofthe material. Also, in U.S. Pat. No. 4,453,404 assigned to MeadCorporation, there is described a scanning system for determiningstatistical characteristics of sheet material; the patent states thatthe system can monitor the weight basis of sheet material, such aspaper, as the material is being produced. Still further, in U.S. Pat.No. 2,806,373 there is disclosed an apparatus for testing sheet papercomprising at least two detectors that are continuously responsive tothickness and opacity variations. The patent states that, for paperproduced in a given paper mill from given raw material, relationshipsexist between various characteristics of the paper and that a knowledgeof some of the characteristics permits conclusions to be drawn regardingother characteristics; particularly, the patent states that variationsin porosity and moisture can be obtained as algebraic functions ofvariations of thickness and substance.

Still further as to prior art, it may be noted that U.S. Pat. No.3,687,802 describes a method and system for controlling the moisturecontent, mullen, and basis weight of paper by measuring each anddeveloping appropriate control signals for adjusting a papermakingmachine so that the desired measurements are approximated. Also, U.S.Pat. No. 3,936,665 discloses a sheet material monitoring apparatusincluding sensing gauges and a computer for determining a data profileacross the sheet material. According to the patent, the monitored datamay be used to provide information to control a sheet-making process toobtain a desired characteristic of the sheet material. Further, thepatent teaches the data profile of a characteristic of a sheet is to beobtained without using scanning gauges.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

Generally speaking, a primary object of the present invention is toprovide improved systems and methods for continuously determiningstrength properties of traveling continuous webs of paper sheet materialduring manufacture on a papermaking machine without destructive testing.

More particularly, an object of the present invention is to provideimproved on-line systems and methods for non-destructively detectingprocess measurement proxies for strength properties of paper sheetmaterials during manufacture, which proxies can be used, for example, tocontrol changes in papermaking processes to selectively vary thestrength of paper sheet material being produced.

In accordance with the preceding objects, the present inventiongenerally provides a non-destructive process and system for continuouslydetermining the strength of paper sheet material during manufacturebased upon detection of process measurement proxies for at least four ofthe following properties relating to sheet strength: the strength ofindividual fibers, the length distribution of fibers, the quantity offibers, the distribution of fibers, the orientation of fibers, thenumber of bonds between fibers, and bond strength. After proxy measuresfor at least four of the properties are detected, statisticalcomputations are made to correlate the proxy measures with paperstrength. Also, operation of the papermaking machine can be controlledbased upon changes in the detected proxies to adjust the strength of theproduced sheet material.

In a preferred embodiment of the present invention, a process proxymeasure for the strength of individual fibers is stress applied to asheet while drying. Such stresses are detected by, for example, anon-destructive scanning-type device that includes support means forsupporting one side of the traveling sheet about a localized unsupportedarea, deflecting means that displace the sheet within the unsupportedarea, force sensors for detecting forces related to the force with whichsaid sheet is deflected within said localized area, and displacementsensors for detecting the distance the sheet is deflected within thelocalized area.

As will become clear from the following description, the methods andsystems of the present invention provide substantial advantages in theart of non-destructive detection of strength properties of travelingcontinuous webs of paper sheet material during manufacture. Additionaladvantages of the present invention can be ascertained by reference tothe following description and attached drawings which illustrate thepreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram for a process according to thepresent invention;

FIG. 2 is a pictorial view of an example of a scanning-type sensingdevice for use in a process and system according to the presentinvention to continuously determine strength of paper sheet material;

FIG. 3 is a side view, partially in cross-section, of the sensing deviceof FIG. 2;

FIG. 4 is a plan view of the lower portion of the sensing device of FIG.3 taken along the plane of the line 4--4 in FIG. 3 for viewing in thedirection of the arrows;

FIG. 5 is a diagram that will be used to assist in explaining operationof the sensing device of FIGS. 2 and 3;

FIG. 6 is a graph that will be used to further assist in explaining theoperation of the sensing device of FIG. 3;

FIG. 7 is a diagram that illustrates the interrelationship ofmeasurements obtained by various sensors, including the sensing deviceof FIGS. 2 and 3, for determination of paper sheet strength in terms ofthe process outlined in FIG. 1; and

FIG. 8 is a block diagram of a system employing process measurementproxies to provide output signals to control a papermaking machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A process for determining paper strength, as generally outlined in FIG.1, is premised upon identification and detection of process measurementproxies or properties for basic physical properties that determine paperstrength. More particularly, based upon the characterization of papersheets as dried mats of fiber, basic strength properties of paper areclassified into two broad categories: properties inherent to fibers, andproperties inherent to structural arrangements of fibers. Propertieswithin the first category include the strength of individual fibers andtheir length distribution. Properties within the second category includethe quantity of fibers, their distribution or "formation", theirorientation, the number of bonds between fibers, and the strength of thebonds. Still speaking generally, sensors are used in the processoutlined in FIG. 1 to provide outputs indicative of process measurementproxies for usually at least four of the basic physical properties thatdetermine paper strength. Consequently, implementation of the process ofFIG. 1 provides simplification and rationalization of the art ofdetermining paper strength in a generally continuous manner while papersheet materials are being produced.

As to sensing the strength of individual fibers, an initial complicationis that direct measurements cannot be made without destroying sheetmaterial. This complication, however, is overcome in the process of FIG.1 by detecting process parameters that are causally correlated with thestrength of individual fibers. More particularly, it has been found thatthe strength of individual fibers primarily depends on fiber species,pulping processes, and stresses applied to sheets while drying. As tofiber species, it has been found that softwoods produce stronger paperthan hardwoods, but that paper strength does not substantially vary withparticular softwood species. (See Setterholm, V. C., and Chilson, W. A.,TAPPI 48, Noll: 634-640, Nov. 1965.) Thus, according to the processoutlines in FIG. 1, it is assumed that the ratio of hardwood pulp tosoftwood pulp provides an indication of paper strength dependent onfiber species. Accordingly, a process measurement proxy for individualfiber strength is provided by tracking the ratio of hardwood to softwoodin a pulp, usually through flow measurements.

Further as to indirect measures of the strength of individual fibers, itis known that differences in pulping processes affect fiber strength.For example, see Mardon, J., et al, "Stock Quality Factors AffectingPaper Machine Efficiency", Technical Section CPPA, Montreal, 1972,wherein it is reported that increasing chemical pulp in newsprint paperincreases strength and the elastic modulus of the newsprint. Typicalpulping processes include groundwood, thermo-mechanical, and kraftprocesses. Many common paper materials, including newsprint, areproduced by mixing pulps produced by two or more pulping processes.Thus, another process measurement proxy for individual fiber strength isprovided by detecting the quantity ratio of pulps fed to a papermakingmachine (i.e., the pulp ratio). The pulp ratio can ordinarily bedetected through simple flow monitoring.

Still further as to indirect measures of the strength of individualfibers, it has been shown that stress applied during drying may increasethe strength of individual fibers in sheet material. See C. Kim et al,"The Mechanical Properties of Single Wood Pulp Fibers", Journal ofApplied Polymer Science, Vol. 14, pp. 1549-1561 (1975). This effect ismost likely caused by wet fibrils sliding past one another during dryingto relieve areas of non-uniform stress concentrations. One manifestationof this effect is that, because of mechanically-induced draw orelongation of paper in the machine direction during production, paper isoften anisotropic in the sense that it is stronger in the machinedirection than in the cross direction. As indicated in FIG. 1,machine-direction draw (i.e., elongation) can be detected as a processmeasurement proxy for individual fiber strength. In practice,machine-direction draw is readily detected by conventional velocitysensors or tachometers attached to selected rolls on a papermakingmachine.

Further, it has been found that stresses build in paper sheets duringdrying because of paper shrinkage and machine draw and because ofrestrictions on physical movement of the sheets. It has also been foundthat the cross-direction elastic moduli decrease from the center towardthe edges of sheets on papermaking machines. Such findings form a basisfor use, in the process outlined in FIG. 1, of measurements related toelastic moduli as process measurement proxies for stress. A sensordevice for detecting the cross-direction and machine direction elasticmoduli is described in conjunction with FIGS. 2 through 6.

Referring still to the process in FIG. 1, the second property within thefiber characteristics category affecting the strength of paper sheetmaterials is fiber length distribution. This property is important tosheet strength because it has been shown that long fibers produce morebonds per fiber than short fibers. One process measurement proxy forfiber length distribution is vacuum on a couch roll of a papermakingmachine. An explanation for the basis for this measurement proxy is thatsheet porosity, as indicated by couch roll vacuum, decreases with fiberlength because short fibers more frequently plug open areas in sheetsthan long fibers. Another measurement proxy for the length distributionof fibers is optical scattering. An explanation for the basis for thismeasurement proxy is that light incident on sheet materials is scatteredmore by short fibers than long fibers. In practice, optical scatteringcan be detected by various conventional infrared scanning devices suchas the infrared Measurex Moisture Sensor manufactured by MeasurexCorporation of Cupertino, Calif.

Measurement of properties within the category of structural arrangementof fibers will now be described in terms of the process outlined inFIG. 1. One of the properties within this category is quantity offibers. Identification of this property as affecting sheet strength isbased upon observations indicating that the strength of fibrous sheetsis generally proportional to the number of fibers within the sheets. Asindicated in FIG. 1, a process measurement proxy for the quantity offibers in a sheet is provided by detecting the dry basis weight of thesheet. Dry basis weight is normally defined as the weight per unit areaof sheet material excluding moisture, and is usually stated in units ofgrams per square meter; thus, in the art of papermaking, dry basisweight is equivalent to the weight of dry material, primarily fibers,comprising a given area of a paper sheet. Related measurement parametersare basis weight and moisture content of a sheet per unit area; suchparameters are related by the fact that, for a given area of a sheet,dry basis weight equals basis weight minus moisture content.

Dry basis weight can be determined by using a Measurex Basis WeightSensor in conjunction with a Measurex Moisture Sensor and a MeasurexX-ray Ash Sensor; the latter two devices measure moisture and fillercontent, respectively. With such measurements, dry basis weight can becalculated by subtracting the moisture and filler content from totalbasis weight. Also, U.S. Pat. No. 4,289,964, assigned to IntecCorporation, suggests that beta ray gauges can scan across a travelingweb in the cross direction to determine basis weight. Further, a devicefor measuring dry basis weight is taught in U.S. patent application Ser.No. 902,225, filed Aug. 29, 1986, now U.S. Pat. No. 4,767,935 andcommonly assigned to Measurex Corporation of Cupertino, Calif. Thedevice described in the application optically measures dry basis weightof sheet materials by reflecting rays of light onto one surface of atraveling paper web and then detecting two distinct light wavelengthstransmitted through the web.

In the sheet-making art, the distribution of fibers within a sheet isreferred to as "formation." Formation can be related to sheet strengthbecause it is known that sheets with different fiber formations havesubstantially different strengths and that sheets with non-uniform fiberformations are weakest at areas, called "thin spots", where fiberconcentrations are minimal. Generally speaking, a process measurementproxy for fiber formation can be obtained by optically detecting thetransmission of collimated light through a traveling sheet using opticalsensors that are relatively insensitive to moisture variations; examplesof suitable optical sensors include ones that measure the transmissionthrough sheet material of highly collimated light at narrow bands ofwavelengths about 1.3 microns. With such optical sensors, thin spots areidentified as transmission maxima.

An alternative process measurement proxy for fiber formation is theratio of jet to wire speed, where jet speed is the speed at which pulpexits a head box. The justification for this measurement is thatturbulence destructive to fiber floc formation is created at jet speedsslower than wire speeds. Wire speed also affects formation becauseincreases in wire speed normally require increases in jet speed. Jetspeed can be detected from conventional measurements of head pressure,and wire speed can be detected by conventional tachometers connected toappropriate rolls in a papermaking machine.

At this juncture, it may be noted that U.S. Pat. No. 3,435,242 disclosesa device that is said to inspect the formation of fibers in sheets ofpaper. The device includes a plurality of narrow-beam photodiodes thatare placed proximate to the material to be tested, a narrow-beamdetector, and an instantaneous ratio computer to provide output signalsrepresenting the structural formation of the tested material. Also, inthe publication "Pulp & Paper" (Aug. 1985, p. 163) there is described adevice called the Lippke sensor to monitor sheet formation or fiberorientation at the wet end of a papermaking process; the Lippke sensoris said to employ a laser light beam and a two-dimensional photodetectorto detect the light scattering differential between randomly orientedfibers and fibers oriented parallel to one another.

Further with regard to FIG. 1, the detection of geometrical orientationsof fibers will be described. In this regard, it should be understoodthat sheet strength has been found to be greater in the direction oforientation of the majority of fibers within a sheet than in otherdirections. To provide process measurement proxies for fiberorientations, a system comprising two microwave moisture sensors can beutilized with one of the sensors oriented in the machine direction andthe other in the cross direction. Also, in U.S. Pat. No. 3,807,868assigned to Valmet Oy, there is described a method for determination offiber orientation in paper including the steps of detecting polarizedlight at right angles to the plane of the paper, reflecting the light intwo planes at right angles to each other, and forming two quantitiesbased upon the reflected light to provide an index value for theanisotropy of fiber orientation. Further, in an article by Z. Koran et.al. entitled "Network Structure and Fiber Orientation in Paper", TAPPIJournal, May 1986 (pp. 126-128), there is described a method fordetecting fiber orientation in paper by X-ray diffraction and zero-spantensile testing. Notwithstanding the foregoing, however, it should bementioned that fiber orientation is normally a function of head boxdesign and, although headbox designs can vary substantially from machineto machine, the effect of headbox design on fiber orientation isgenerally constant for a given machine.

Still another property affecting paper strength is the number of bondsformed by fibers within a sheet. Because the number of bonds within apaper sheet generally increases as fibers are packed more denselytogether, density measurements can provide a process measurement proxyfor the number of bonds between fibers. For purposes of the processoutlined in FIG. 1, density measurements should be made at the wet endof a papermaking process because, at the dry end, calendering increasesdensity but may weaken the sheet material by mechanically breaking downfibers. In practice, density can be directly detected on-line usingconventional sensors for determining basis weight and caliper.

As to the strength of bonds between fibers, it has been found that bondstrength primarily depends on fiber species, pulping processes,additives, pressure, and moisture. The relationship of bond strength tofiber species is essentially the same as the relationship of individualfiber strength to fiber species, namely that stronger bonds are providedby softwoods than hardwoods but that bond strength is substantiallyindependent of particular hardwood species or softwood species so longas the ratio of hardwood pulp to softwood pulp is maintained constant.Thus a process measurement proxy for individual bond strength isprovided by monitoring the ratio of hardwood to softwood in feed pulp,usually through flow monitoring devices. Also, as in the case of fiberstrength, bond strength varies as a function of the pulp ratio of feedto a papermaking machine; again, this proxy measure can be detected byconventional flow monitoring devices.

In papermaking processes, additives such as gum or starch are sometimesused to increase bond strenght. When such additives are used, thequantity of the additives can be measured by flow measurement devicesand calibrations can be made to reflect the effectiveness of theadditives on the strength of bonds between fibers.

Further as to the strength of bonds between fibers, it is known that wetpressing increases bond strength by increasing the contact area betweenfibers and allowing more bonds to form. In practice, most wet pressingis accomplished by crown rolls that operate at constant pressure. Thus,although wet pressing can affect bond strength, such processing is notusually variable and, therefore, does not require constant monitoring asa factor affecting sheet strength.

The moisture content of fibrous sheet material also affects bondstrength and is primarily determined by dryer temperature and wirespeed. In practice, the moisture content of sheet materials is readilymeasured on-line with conventional moisture sensors such as theaforementioned Measurex moisture sensor. As indicated in FIG. 1, thedetected moisture content of sheet material can be used to indicate thestrength of fiber bonds and, hence, the strength of the sheet material.

The relative contribution of each of the above-mentioned processmeasurement proxies to the strength of a particular paper sheet materialgenerally depends upon characteristics of the papermaking machine inwhich the sheet material is formed. For purposes of process control ofan individual machine, some of the process measurement proxies can oftenbe considered to be invariant. For instance, although orientation offibers affects paper strength and can be determined by head box design,the effects of head box design are usually constant during operation ofmodern papermaking machines and, therefore, can usually be accounted forby instrument calibration and need not be continuously measured.

FIG. 7 provides a diagram of the interrelationship of processmeasurement proxies and parameter sensors to the determination of papersheet strength. It should be noted that the seven properties listed inFIG. 7 are the same as the ones discussed with respect to FIG. 1.Further, FIG. 7 indicates types of sensors that can be used to provideprocess measurement proxies that determine the seven properties. Forexample, an infrared (IR) moisture sensor can be employed to detect themoisture content of a traveling sheet and, hence, to provide a measureof bond strength between fibers. It should be noted that a particulartype of sensor can be used to provide more than one process measurementproxy; for example, an IR moisture sensor can be used to indicate bothmoisture content and dry fiber weight.

Referring now to FIG. 2, there is shown a scanning station, generallydesignated by the number 111, that extends across a paper web 113 in thecross direction and that includes a sensor for detecting stress in web113. In the particular embodiment of scanning station 111 chosen forillustration, web 113 passes horizontally between a pair of stationaryparallel beams 121 and 122 that are mounted to extend transverselyacross the web parallel to its opposite faces. Depending upon thepapermaking machine, beams 121 and 122 can range in length from about100 inches to about 400 inches. Normally, scanning station 111 islocated at a position on a papermaking machine where paper motion isrelatively stable and not subject to substantial variations, such asflutter.

As further shown in FIG. 2, scanning station 111 includes travelingcarriage devices 123 and 124 that are mounted on upper and lower beams121 and 122, respectively to scan web 113. (In FIG. 2, web 113 is shownwith a cut out area so that the lower carriage device 124 is notobscured.) A conventional drive mechanism, not shown, is provided todrive carriage devices 123 and 124 back and forth along beams 121 and122. In operation, the drive mechanism functions to operate carriagedevices 123 and 124 in synchronization, with one always aligned oppositethe other.

Also in connection with FIG. 2, it should be noted that upper carriagedevice 123 carries one part of a sensor, generally designated by thenumber 125, and that lower carriage device 124 carries another part ofthe same sensor; the complete sensor 125 is shown in FIG. 3. Generallysimilar sensors are shown in the following U.S. Patent Applicationsassigned to Measurex Corporation of Cupertino, Calif.: Ser. No. 730,406filed May 2, 1985; Ser. No. 784,213 filed Oct. 4, 1985 (now U.S. Pat.No. 4,864,581); and U.S. patent application filed July 18, 1986 as acontinuation-in-part of application Ser. No. 784,213, now abandoned andrefiled as Ser. No. 195,364, filed May 13, 1988 (now U.S. Pat. No.4,866,984).

Referring now to FIG. 3, the structure of the upper portion of sensor125 will be described first. The upper portion comprises a clevis-likebracket 128 having U-shaped legs 130. Bracket 129 is coupled to carriagedevice 123 by a horizontal pivot pin 129 that allows the bracket topivot vertically relative to the surface of web 113. A wheel 131 ismounted to rotate freely on a horizontal axle 133 extending betweenU-shaped legs 130 and is dimensioned to ride on the surface of web 113.Pivotal motion of bracket 128 in the vertical direction is limited by anair cylinder 135 that is pivotably linked between carriage 123 and thebracket. Air cylinder 135 is normally provided with compressed air atsufficient pressure to keep wheel 131 positioned against the surface ofweb 113 at a generally fixed location in the vertical direction.

The lower portion of sensor 125, as shown in FIGS. 3 and 4, generallyincludes at least two pairs of force sensors, designated in the drawingsas pair 143A and 143B and pair 144A and 144B, and a displacement sensor145. The force sensors and the displacement sensor are all rigidlyconnected to a plate 146 fixed to lower carriage device 124. Further, asshown in FIGS. 3 and 4, the lower portion of sensor 125 includes pairsof contact members 147A and 147B, and 148A and 148B, respectively, whichare so named because they are in contact with the undersurface of web113. In the illustrated embodiment, contact members 147A and 147B aremounted at the free ends of an associated pair of flexible cantilevermembers 149A and 149B, respectively, whose opposite ends arestationarily mounted to plate 146 by spacers 150. Similarly, contactmembers 148A and 148B are mounted to the free ends of an associated pairof flexible cantilever members 151A and 151B whose other ends arestationarily mounted to plate 144 by spacer members (not shown). Thepair of contact members 147A and 147B are aligned in the machinedirection and will be referred to herein as the machine-direction pair;likewise, the pair of contact members 148A and 148B are aligned in thecross-direction, and will be referred to herein as the cross-directionpair.

As best shown in FIG. 3, the machine-direction pair of contact members147A and 147B are connected to respective force sensors 143A and 143B bypin members 152A and 152B, respectively, such that displacement of thecontact members exerts forces on the sensors via vertical movement ofthe pin members. It should be understood that the cross-direction pairof movable contact members 148A and 148B are similarly connected toforce sensors 144A and 144B, respectively. In practice, the forcesensors are conventional piezo resistance devices or strain gauges thatprovide electrical output voltages proportional, preferably linearly, tothe force exerted on the cells. Also in practice, displacement sensor145 is a conventional proximity sensor that provides output voltagesthat are proportional, again preferably linearly, to the distancebetween the displacement sensor and the surface of web 113.

Operation of sensor 125 of FIGS. 2 through 4 will now be generallydescribed. Initially, it should be assumed that sensor 125 is assembledsuch that both the machine-direction pair and the cross-direction pairof contact members are located to press against the undersurface of web113 and so that wheel 131 is in contact with the surface of web 113 at apoint generally midway between the contact members of the respectivepairs and generally directly above displacement sensor 145. Also, forpurposes of understanding the preferred mode of operation of sensor 125,it should be assumed that cylinder 135 is pressurized sufficiently thatwheel 131 can be considered to have a fixed location in the verticaldirection. In such an assembly, force sensors 143A and 143B function todetect the amount of force with which traveling web 113 presses againstmovable contact members 147A and 147B, and displacement sensor 145detects changes in the position of the surface of web 113 relative toplate 146.

Further operation of sensor 125 will be explained in conjunction withFIG. 5. In that diagram, dimension "d" indicates the horizontal distancefrom the point of contact of wheel 131 with the surface of web 113 tothe point of contact of one of the movable contact members, say member147B, with web 113. Dimension "z" indicates the vertical distance thatthe surface of web 113 is displaced by wheel 131. Although dimension "z"is constant ideally, it will vary somewhat in actual operation ofscanning sensor 125 due to mechanical flexure of the sensor. Tocompensate for such variations, the value of "z" is monitored bydisplacement sensor 145 and a signal whose amplitude is representativeof the distance "z" is provided at the output of the sensor. Knowing thedetected values for "z" and "d", dimension "h" can be determined.Geometrically, dimension "h" represents the hypotenuse of a trianglewhose legs are "d" and "z", respectively, and can be understood to begenerally collinear with the surface of web 113 that extends from theselected contact member to the point of contact with wheel 131.

For a given draw of web 113, the stress in web 113 along distance "h"can be detected as a function of the force exerted by web 113 on one ofthe force sensors 143A, 143B, 144A or 144B. Thus, if variable "S"represents the stress in web 113 along distance "d" and if "F"designates the force sensed by one of the sensors when wheel 131 is incontact with web 113, the relationship between S and F can be resolvedby a force triangle and can be expressed as:

    S sin θ=F

where θ is the angle between "d" and "h" as indicated in FIG. 5. Theabove equation can be rearranged to be:

    S=F/ sin θ.

Since sin θ is nearly equal to tan θ for small angles, the relationshipbetween S and F can be expressed by the approximation:

    S=F/ tan θ.

Because tan θ is the ratio of "z" to "d", the stress S in web 113 can beexpressed as: ##EQU1##

Thus, for practical purposes, the stress in web 113 can be understood tobe a function of three measured properties: the displacement (z) of web113, the placement (d) of wheel 131 relative to a force sensor, and theforce (F) detected by one of the force sensors.

In practice, the forces applied to the machine-direction pair of forcesensors 143A and 143B are not necessarily the same as the forces appliedto the cross-direction pair of force sensors 143C and 143D because ofdifferences in stresses and tension in the machine direction and thecross direction. As a result of these differences, different strengthscan be determined for web 113 in the cross direction and in the machinedirection. It may also be noted that, in the cross direction, a sheet isrestrained but is not usually subject to substantial draw. Nevertheless,sensor 125 can provide meaningful strength measurements in the crossdirection because it, in essence, induces localized stress bydisplacement of the surface of the sheet by wheel 131.

An alternative mode of operation of sensor 125 will now be describedwherein cylinder 135 is pressurized and controlled such that wheel 131is movable and presses downward against web 113 with a generallyconstant force such that web 113 is displaced only a relatively smalldistance toward displacement sensor 145 when web 113 is under normaltension. In such a mode of operation, displacement sensor 145 againdetects changes in position "z" of the surface of web 113 relative toplate 146 and, generally speaking, position "z" varies as a function ofthe stress "S" of web 113. Thus, under practical operating conditions,the stress in web 113 can again be expressed as a function of threemeasured properties: the displacement (z) of web 113, the placement (d)of wheel 131 relative to a selected force sensor, and the force (F)detected by a selected one of the force sensors.

The stress measurement "S", as detected through the above-describedoperation of sensor 125, can be used to predict strength properties ofweb 113 in conjunction with the detection of process measurement proxiesaccording to FIG. 1. The relationship of stress "S" to the processmeasurement proxies will now be explained in terms of the graph in FIG.6, whose vertical axis represents stresses applied to a given area ofweb 113 by sensor 125 and whose horizontal axis represents the strain onthe same section of the web. Here again, strain can be interpreted to beelongation or "draw" of web 113 and can be measured in the machinedirection by continuously monitoring the velocity of rollers over whichthe web travels. In the cross direction, draw is normally constantacross a sheet and, therefore, need not be monitored.

In FIG. 6, the curves A, B and C illustrate stress-strain relationshipsfor various types, or grades, of paper. Thus, curve "A" depicts therelationship between stress that is applied to a particular type ofpaper and the elongation of that paper. Likewise, curve "B" representsthe relationship between stress applied to another type of paper andelongation of that paper. Comparing paper types or grades A and B, itcan be seen that a given stress on paper B will create more elongationthan the same stress on paper A. The end points of the curves A, B and Care failure points, (i.e., points at which the papers break) and,therefore, indicate the strengths of the paper types. Again comparingpaper types A and B, it can be seen that paper type B is weaker thanpaper type A since it breaks at a lower applied stress. Further withregard to FIG. 6, it should be understood that the slope of the curvesA, B and C represent Young's modulus for various grades of paper; forexample, the slope of curve A at point "a" is Young's modulus for thatgrade of paper.

The failure points for the paper grades described by the curves in FIG.6 are normally determined empirically, which is to say by standardizedlaboratory methods. In practice, a reliable correlation can often beobtained between laboratory results for various standarized methods. Forexample, highly reliable correlations have been obtained forstandardized laboratory tests such as standardized tensile, STFIcompressive and Mullen burst pressure tests.

Because samples of the same grade of paper have generally the samestress-strain relationship, it can be understood from FIG. 6 that gradecan be identified for a given paper if stress and strain are measured.For example, if stress point a' and strain point a" are measured for agiven paper, the point "a" can be determined and, further, the samplecan be identified as having stress-strain properties unique to the papergrade depicted, in such an instance, by curve "A".

It should be further understood that the failure points for the papergrades as depicted by curves A, B and C in FIG. 5 are not necessarilyconstant for a given paper grade but, instead, can vary depending uponthe physical properties discussed in conjunction with FIG. 1 (i.e.,fiber characteristics and the structural arrangement of fibers). Inpractice, for a given papermaking machine and paper grade, functionalrelationships of sheet strength to the process measurement proxies canbe determined using empirical methods and techniques of multipleregression analyses as will be discussed in the following.

FIG. 8 shows a system that provides output signals for control purposesbased upon the process measurement proxies. In FIG. 8, signals thatrepresent at least four of the seven process measurement proxies areindicated as being fed to a computer 201. For the sheet material beingproduced, computer 201 also receives input indicating strengthproperties as determined by standard quality laboratory techniques.Computer 201 can be understood to be a conventional digital computerthat is programmed with algorithms for multiple regression analyses.Because the process measurement proxy information is normally in analogform, such information must usually be digitized for use by computer201. Such digitizing can be performed by conventional analog-to-digitalconverter devices, not shown. With the inputs indicated and whenconventionally programmed with correlation and regression algorithms,digital computer 201 operates to determine correlations between theprocess measurement proxies and laboratory-determined strength values atselected locations in the cross direction of web 113.

Once functional relationships employing the process measurement proxiesare determined by computer 201 of FIG. 8, papermaking can be controlledby monitoring the process measurement proxies. For example, after aparticular grade of paper is identified, sensor 125 operates inconjunction with other selected sensors to provide process measurementproxies as digital input signals to computer 201 which, ultimately,provides output signals that indicate changes in the strength of web113. That is, computer 201 can provide output signals representative ofdeviations of the calculated strength of the sheet material beingproduced from the desired strength value at each of the cross sectionlocations. The output signals can also be employed to control thepapermaking machine by causing adjustments to be made at various crosssection locations.

Although the system of FIG. 8 can provide meaningful measures of thestrength of web 113 using fewer than all seven of the properties listedin FIG. 7, independent process measurement proxies for at least four ofthe properties normally must be obtained to provide adequate accuracyfor control purposes in processes involving substantially changingconditions. To obtain functions of the process measurement proxies,conventional techniques of standard regression analysis are employed.One form of such a regression equation is, for example, the following"Mullen" strength equation:

    S.sub.mu =A*(JW)+B*(VAC)+C*(S)+D*(MOI)+E*(% CHEM)+F(BW)+G*(ρ)+H

where:

S_(mu) is the "Mullen" strength of the paper;

A, B, C, D, E, F, G, and H are regression fit constants;

JW=jet-wire speed;

VAC=couch vacuum;

S=tension in the web;

MOI=percent moisture;

% Chem=percent chemical pulp;

BW=basis weight; and

ρ=density.

The values of the constants A through H in the preceding equationgenerally depend on the particular papermaking machine and grade ofpaper. If paper grade is changed substantially, then the constants mustnormally be recalculated.

According to the preceding equation, Mullen strength can be calculatedat selected cross-direction locations by scanning across a web duringproduction. A set of Mullen strength data for a complete scan of a webprovides a "profile" of the web. For process control purposes, it isnormally important to identify the cross direction location of eachcomponent measurement of a profile.

In practice, strength profile measurements obtained in the mannerdescribed above provide control advantages during startups, gradechanges, and process upsets. Also, the strength profile measurements canbe used to reduce sheet variations in the machine direction duringsteady-state operation by, for example, providing control signals toadjust draw in the machine direction. As another example, such strengthprofile measurements can be used during newsprint production to provideadjustments for the feed mixture ratio of groundwood to chemical pulp tocontrol paper strength and to increase paper production rates. Also, theabove-described system and method can provide arbitrary indexes ofstrength that allow strength comparisons between paper productsindependently of standardized systems.

Although the present invention has been described with particularreference to preferred embodiments, such disclosure should not beinterpreted as limiting. Various alterations and modifications to thepreferred embodiments will no doubt become apparent to those skilled inthe art after having read the preceding disclosure. It is intended thatthe appended claims be interpreted as covering all alternativeembodiments and equivalents as fall within the spirit and scope of thepresent invention.

I claim:
 1. A process for detecting the strength of continuous papersheet material during manufacture by detecting parameters indicative ofat least four of the following properties: (a) the strength ofindividual fibers, (b) the length distribution of fibers, (c) thequantity of fibers, (d) the distribution of fibers, (e) the orientationof fibers, (f) the number of bonds between fibers, and (g) the bondstrength of fibers, the process comprising the steps of:(i) sensing theparameters for at least four of said properties during manufacture ofpaper sheet material in a papermaking machine; (ii) for the selectedpapermaking machine and paper grade, calculating correlations of said atleast four parameters with laboratory tests of paper strength; and (iii)during production of paper on the papermaking machine, nondestructivelysensing the at least four parameters and, based upon the determinedcorrelations, calculating the predicted strength of paper being producedby the papermaking machine.
 2. A process as defined in claim 1, whereinthe step of nondestructively sensing the parameter indicative of thestrength of individual fibers includes detecting the ratio of softwoodto hardwood in pulp fed to the papermaking machine.
 3. A process asdefined in claim 1, wherein the step of nondestructively sensing theparameter indicative of the strength of individual fibers includesdetecting the ratio of different pulps fed to the papermaking machine.4. A process as defined in claim 1, wherein the step of nondestructivelysensing the parameter indicative of the strength of individual fibersincludes detecting draw in the machine direction.
 5. A process asdefined in claim 4, wherein the step of nondestructively sensing theparameter indicative of the strength of individual fibers furtherincludes detecting stress applied to drying fibers.
 6. A process asdefined in claim 5 wherein the step of detecting stress applied todrying fibers includes detecting the elastic modulus in the crossdirection and in the machine direction.
 7. A process as defined in claim1, wherein the step of nondestructively sensing the parameter indicativeof the length distribution of fibers includes detecting couch rollvacuum on the papermaking machine.
 8. A process as defined in claim 1,wherein the step of nondestructively sensing the parameter indicative ofthe length distribution of fibers includes detecting optical scatteringof infrared light directed against the sheet material.
 9. A process asdefined in claim 1, where in the step of nondestructively sensing theparameter indicative of the quantity of fibers includes detecting thedry basis weight of paper sheet material on the papermaking machine. 10.A process as defined in claim 1, wherein the step of nondestructivelysensing the parameter indicative of the distribution of fibers includesdetecting the transmissivity of the paper sheet material.
 11. A processas defined in claim 1, wherein the step of nondestructively sensing theparameter indicative of the distribution of fibers includes detectingthe jet to wire speed ratio for the papermaking machine.
 12. A processas defined in claim 1, wherein the step of nondestructively sensing theparameter indicative of the orientation of fibers includes detectingmoisture in both the machine direction and cross direction on thepapermaking machine.
 13. A process as defined in claim 1, wherein thestep of nondestructively sensing the parameter indicative of the numberof bonds between fibers includes detecting the density of the papersheet material on the papermaking machine.
 14. A process as defined inclaim 13 wherein density is detected by measuring basis weight andcaliper of sheet material being produced on the papermaking machine. 15.A process as defined in claim 1, wherein the step of nondestructivelysensing the parameter indicative of the strength of bonds between fibersincludes detecting the ratio of different pulps fed to the papermakingmachine.
 16. A process as defined in claim 1, wherein the step ofnondestructively sensing the parameter indicative of the strength ofbonds between fibers includes detecting the ratio of softwood tohardwood in pulp fed to the papermaking machine.
 17. A process asdefined in claim 1, wherein the step of nondestructively sensing theparameter indicative of the strength of bonds between fibers includesdetecting the moisture content of the sheet material being produced bythe papermaking machine.
 18. A process as defined in claim 1, whereinthe step of nondestructively sensing the parameter indicative of thestrength of bonds between fibers includes detecting the quantity ofadditives in pulp fed to the papermaking machine.
 19. A process asdefined in claim 6 wherein the step of detecting the elastic modulus isaccomplished by a scanning-type sensor device that includes supportmeans for supporting one side of said traveling sheet about a localizedunsupported area, deflecting means for displacing said sheet within saidlocalized unsupported area, first sensing means for detecting forcesrelated to the force with which said sheet is deflected within saidlocalized area, and second sensing means for detecting the distance thesheet is deflected within the localized area.
 20. A process as definedin claim 19 further including the step of correlating output signalsfrom said first and second sensing means with a standardized measure ofstrength of the sheet material at selected locations in thecross-direction.